Alloyed hot-dip galvanized steel sheet and method of manufacturing the same

ABSTRACT

Provided is an alloyed hot-dip galvanized steel sheet including a base steel sheet, the base steel sheet containing a given amount of C, Si, Mn, and other elements. The alloyed hot-dip galvanized steel sheet is provided with an alloyed hot-dip galvanized layer on a surface of the base steel sheet, the alloyed hot-dip galvanized layer containing, in mass %, Fe: more than or equal to 5% and less than or equal to 15%, and having a thickness of more than or equal to 3 μm and less than or equal to 30 μm. The alloyed hot-dip galvanized steel sheet includes an A layer immediately under the surface of the base steel sheet, the A layer being formed in the base steel sheet and having a thickness of more than or equal to 2 μm and less than or equal to 20 μm from the surface of the base steel sheet, containing more than or equal to 50 vol % of a ferrite structure, and containing more than or equal to 90 mass % of unoxidized Fe, less than or equal to 10 mass % of a total of contents of oxides of Fe, Si, Mn, P, S, and Al, and less than 0.05 mass % of C.

This application is a Divisional of U.S. application Ser. No.14/438,503, filed Apr. 24, 2015, which is the U.S. National Phase ofPCT/JP2013/079858, filed Nov. 5, 2013, which claims priority under 35U.S.C. 119(a) to Japanese Patent Application No. 2012-244274, filed Nov.6, 2012, the contents of all of which are incorporated by reference, intheir entirety, into the present application.

TECHNICAL FIELD

The present invention relates to an alloyed hot-dip galvanized steelsheet and a method of manufacturing the same. In more detail, presentinvention relates to a high-strength alloyed hot-dip galvanized steelsheet having a tensile strength of 590 MPa or more, including an alloyedhot-dip galvanized layer having excellent wettability of plating andadhesion of plated layer which can be applied as a material used in anautomotive field, a household appliance field, and a building materialfield, and to a method of manufacturing the same.

BACKGROUND ART

In materials used in an automotive field, a household appliance field,and a building material field, a surface treated steel sheet which isimparted with corrosion prevention is being used. In particular, analloyed hot-dip galvanized steel sheet which can be produced at low costand is excellent in corrosion prevention is being used.

In general, the alloyed hot-dip galvanized steel sheet is manufacturedby the following method using a continuous hot-dip galvanizing plant.First, a slab is hot rolled, cold rolled, or heat treated to obtain athin-gauge steel sheet. The thin-gauge steel sheet is degreased and/orpickled in a pretreatment step for the purpose of cleaning the surfaceof the base steel sheet or, omitting the pretreatment step, is heated ina preheating furnace to burn off the oil on the surface of the basesteel sheet, then is subjected to heating and recrystallizationannealing. The atmosphere at the time of performing therecrystallization annealing is an Fe reducing atmosphere since at thetime of the later plating treatment, Fe oxides would obstruct thewettability of the plated layer and the base steel sheet or the adhesionof the plated layer and the base steel sheet. After therecrystallization annealing, without contacting the air, the steel sheetis continuously cooled to a temperature suitable for plating in an Fereducing atmosphere and dipped in a hot-dip galvanizing bath for hot-dipgalvanization. After the hot-dip galvanization, the amount of adhesionof the plating is controlled by immediately performing wiping bynitrogen gas. After that, the heating is performed to thereby conduct anFe—Zn alloying reaction, and in this way, the alloyed hot-dip galvanizedlayer is formed on the base steel sheet.

In recent years, in particular in the automotive field, to achieve boththe function of protecting the passengers at the time of collision andlighter weight aimed at improvement of the fuel efficiency, use of ahigh-strength steel sheet which is made higher in strength of the basesteel sheet by inclusion of elements which are relatively inexpensive,such as C, Si, and Mn, has been increasing. Regarding the strength, thesteel sheet having a tensile strength of 590 MPa or more is mainly used.

However, in the high-strength alloyed hot-dip galvanized steel sheetincluding Si and Mn, Si and Mn are elements which are more easilyoxidizable compared with Fe, so at the time of heating inrecrystallization annealing in a conventional Fe-reducing atmosphere, Siand Mn on the surface of the steel sheet oxidize. Further, Si and Mnwhich thermally diffuse from the inside of the steel sheet oxidize atthe steel sheet surface whereby gradually the Si and Mn oxides becomeconcentrated on the surface. If the Si and Mn oxides concentrate at thesurface, in the process of dipping the steel sheet in the hot-dipgalvanizing bath, contact between the molten zinc and the base steelsheet would be prevented, which would cause a drop in the wettability ofplating and the adhesion of plated layer of the alloyed hot-dipgalvanized layer. If the plating layer deteriorates in wettability,nonplating defects occur and result in defects in appearance and defectsin corrosion prevention. If the adhesion of plated layer deteriorates,peeling of the plating occurs when press forming is performed, andresults in problems including defects in corrosion prevention anddefects in appearance with press scratches and the like.

Further, in the high-strength alloyed hot-dip galvanized steel sheetcontaining C, when C is present in a grain boundary or a grain of thebase steel sheet in the recrystallization annealing, there is a problemin that the reaction between the molten zinc and the steel sheet in theprocess of Fe—Zn alloying reaction after dipping the base steel sheet inthe hot-dip galvanizing bath is inhibited, to thereby deteriorate theadhesion of plated layer. In addition, there is also a problem in thatthe inclusion of C in the alloyed hot-dip galvanized layer after thealloying reaction lowers the ductility of the plating, so that peelingof the plating easily occurs when press forming is performed.

Still further, in the high-strength alloyed hot-dip galvanized steelsheet, the ductility deteriorates with the increase in the strength ofthe base steel sheet, and along therewith, pressing load at the time ofperforming press forming is large, so that the shear stress applied tothe plated layer from a mold at the time of performing formingincreases. Accordingly, there is a problem that the plated layer iseasily peeled from the interface with the base steel sheet, and resultsin problems including defects in corrosion prevention and defects inappearance with press scratches and the like.

As measures for the problems attributed to the concentration of oxidesof Si and Mn at the time of annealing, there have been proposed varioustechniques in the past.

As the technique focusing on suppressing concentration of oxides of Siand Mn, Patent Literature 1 shows a method including performingannealing under an oxidizing atmosphere of Si so that the thickness ofthe oxide film of the steel sheet surface becomes 400 to 10000 Å, thenreducing the Fe in the furnace atmosphere containing hydrogen, andperforming plating. Further, Patent Literature 2 shows a methodincluding oxidizing the Fe on the steel sheet surface, controlling theoxygen potential in the reducing furnace to thereby reduce the Fe andinternally oxidize the Si so as to suppress the concentration of Sioxides on the surface, and then performing plating. However, in thosetechniques, if the reduction time is too long, Si concentrates at thesurface, and if the reduction time is too short, an Fe oxide filmremains on the steel sheet surface. Accordingly, there is the problemthat issues in the plating layer wettability and the plating layeradhesion are insufficiently resolved. In addition, if Fe oxides areformed on the steel sheet surface inside an annealing furnace, the Feoxides are deposited on a roll inside the furnace, and with increase inthe amount of the deposit, there is a problem that roll pickup iscaused, such as defects in appearance with press scratches on the steelsheet.

Patent Literature 3 shows a technique of suppressing the concentrationof oxides of Si and Mn on the surface by raising the oxygen potential inthe atmosphere in an all radiant tube type annealing furnace andinternally oxidizing Si and Mn. Further, Patent Literatures 4 and 5 showmethods including carefully controlling the means and conditions forraising the oxygen potential to suppress the surface concentration ofboth Fe oxides and Si and Mn oxides, and then performing plating.However, none of those techniques are insufficient in suppressing theconcentration of oxides of Si and Mn. Further, since internal oxides ofSi and Mn formed on the surface of the base steel sheet are present inthe vicinity of the surface of the inside of the base steel sheet, thereis a problem that the ductility of the base steel sheet deteriorates andthe press forming cannot be performed. In addition, when a shear stressis applied to the plated layer at the time of performing the pressforming, there is a problem that the plated layer peels from thevicinity of the surface of the inside of the base steel sheet in whichthe internal oxides are present.

Patent Literature 6 shows a method including raising the hydrogenconcentration in the atmosphere in the recrystallization annealing up tothe reducing region in which Fe, Si, and Mn do not oxidize, andperforming plating. However, in this technique, there is a problem inaddition to that the cost of hydrogen becomes immense, that the presenceof C on the surface of the base steel sheet deteriorates the adhesion ofplated layer as described above, and the remaining Si and Mn obstructthe reaction between the plating and the base steel sheet and formoxides of Si and Mn by being reacted with oxides floating on the surfaceof the bath at the time of dipping in the plating bath, so thewettability of plating and the adhesion of plated layer deteriorate.

Further, as a technique for suppressing the concentration of oxides ofSi and Mn, Patent Literature 7, which focuses on causing internaloxidation in advance in the hot rolling step, shows a technique ofcontrolling the oxygen potential in the hot rolling step so as to causeinternal oxidation of Si and using the resultant thin-gauge steel sheetto manufacture a hot-dip galvanized steel sheet in a continuous hot-dipgalvanizing plant. However, in this technique, at the time of the coldrolling step and other rolling, the layer of internal oxidation alsoends up being rolled together, so the internal oxidation layer becomessmaller in thickness and Si oxides end up concentrating on the surfacein the recrystallization annealing process, so there is a problem thatthe wettability of plating and the adhesion of plated layer areinsufficiently improved. Further, there is a problem that oxides of Fe,which are formed simultaneously with internal oxidization of Si in thehot rolling step, cause roll pickup.

Further, the techniques written in Patent Literatures 1 to 7 areinsufficient for solving the problem of the adhesion of plated layerrelated to the deterioration of ductility caused by increase in thestrength of the alloyed hot-dip galvanized steel sheet.

PRIOR ART LITERATURE(S) Patent Literature(s)

-   [Patent Literature 1] JP 555-122865A-   [Patent Literature 2] JP 2001-323355A-   [Patent Literature 3] JP 2008-7842A-   [Patent Literature 4] JP 2001-279412A-   [Patent Literature 5] JP 2009-209397A-   [Patent Literature 6] JP 2010-126757A-   [Patent Literature 7] JP 2000-309847A

SUMMARY OF THE INVENTION Problem(s) to Be Solved by the Invention

The present invention provides a high-strength alloyed hot-dipgalvanized steel sheet including an alloyed hot-dip galvanized layerhaving excellent wettability of plating and adhesion of plated layer ona base steel sheet containing C, Si, and Mn, and a method ofmanufacturing the same.

Means for Solving the Problem(s)

In order to solve the problems, the inventors of the present inventionhave focused on influences on the wettability of plating and theadhesion of plated layer of a content of a ferrite structure, a contentof unoxidized Fe, contents of oxides of Fe, Si, and Mn, and a content ofC in the steel sheet which is immediately under the base steel sheet inparticular, among the alloyed hot-dip galvanized layer and the basesteel sheet in the alloyed hot-dip galvanized steel sheet. Further, asthe method of manufacturing the alloyed hot-dip galvanized steel sheet,the inventors of the present invention have focused on controlling, in acontinuous hot-dip galvanizing plant including a heating furnace and asoaking furnace, a value of a logarithm log(P_(H2O)/P_(H2)) of a valueobtained by dividing a partial water vapor pressure P_(H2O) by a partialhydrogen pressure (P_(H2)) of an atmosphere in each of the heatingfurnace and the soaking furnace, in each of the heating furnace and thesoaking furnace, and have conducted intensive studies. As a result, theinventors of the present invention have found that a high-strengthalloyed hot-dip galvanized steel sheet having excellent wettability ofplating and adhesion of plated layer and having a tensile strength of590 MPa or more can be manufactured, and thus, the present invention hasbeen made.

That is, the gist of the present invention is as follows.

-   [1] An alloyed hot-dip galvanized steel sheet including a base steel    sheet,

wherein the base steel sheet contains, in mass %,

C: more than or equal to 0.05% and less than or equal to 0.50%,

Si: more than or equal to 0.2% and less than or equal to 3.0%,

Mn: more than or equal to 0.5% and less than or equal to 5.0%,

Al: more than or equal to 0.001 and less than or equal to 1.0%,

P: less than or equal to 0.1%,

S: less than or equal to 0.01%,

N: less than or equal to 0.01%, and

the balance including Fe and inevitable impurities,

wherein the alloyed hot-dip galvanized steel sheet is provided with analloyed hot-dip galvanized layer on a surface of the base steel sheet,the alloyed hot-dip galvanized layer containing, in mass %, Fe: morethan or equal to 5% and less than or equal to 15%, and the balanceincluding Zn and inevitable impurities, and having a thickness of morethan or equal to 3 μm and less than or equal to 30 μm, and

wherein the alloyed hot-dip galvanized steel sheet includes an A layerimmediately under the surface of the base steel sheet, the A layer beingformed in the base steel sheet and having a thickness of more than orequal to 2 μm and less than or equal to 20 μm from the surface of thebase steel sheet,

-   -   the A layer containing more than or equal to 50 vol % of a        ferrite structure based on a volume of the A layer and the        balance including inevitable structures, and containing, based        on a mass of the A layer, more than or equal to 90 mass % of        unoxidized Fe, less than or equal to 10 mass % of a total of        contents of oxides of Fe, Si, Mn, P, S, and Al, and less than        0.05 mass % of C.

-   [2] The alloyed hot-dip galvanized steel sheet according to [1],

wherein the base steel sheet further contains one or more of, in mass %,

Cr: more than or equal to 0.05% and less than or equal to 1.0%,

Ni: more than or equal to 0.05% and less than or equal to 1.0%,

Cu: more than or equal to 0.05% and less than or equal to 1.0%,

Nb: more than or equal to 0.005% and less than or equal to 0.3%,

Ti: more than or equal to 0.005% and less than or equal to 0.3%,

V: more than or equal to 0.005% and less than or equal to 0.5%,

B: more than or equal to 0.0001% and less than or equal to 0.01%,

Ca: more than or equal to 0.0005% and less than or equal to 0.04%,

Mg: more than or equal to 0.0005% and less than or equal to 0.04%,

La: more than or equal to 0.0005% and less than or equal to 0.04%,

Ce: more than or equal to 0.0005% and less than or equal to 0.04%, and

Y: more than or equal to 0.0005% and less than or equal to 0.04%.

-   [2] The alloyed hot-dip galvanized steel sheet according to [1] or    [2],

wherein the alloyed hot-dip galvanized layer further contains, in mass%, Al: more than or equal to 0.02% and less than or equal to 1.0%.

-   [4] A method of manufacturing an alloyed hot-dip galvanized steel    sheet including a base steel material, the base steel material    containing, in mass %,

C: more than or equal to 0.05% and less than or equal to 0.50%,

Si: more than or equal to 0.2% and less than or equal to 3.0%,

Mn: more than or equal to 0.5% and less than or equal to 5.0%,

Al: more than or equal to 0.001 and less than or equal to 1.0%,

P: less than or equal to 0.1%,

S: less than or equal to 0.01%,

N: less than or equal to 0.01%, and

the balance including Fe and inevitable impurities,

the method including:

performing casting, hot-rolling, pickling, and cold rolling to therebyproduce the base steel material;

subjecting the base steel material to a hot-dip galvanizing treatment byperforming, using a continuous hot-dip galvanizing plant equipped with aheating furnace and a soaking furnace, an annealing treatment in which atemperature of the base steel material is increased within a range ofhigher than or equal to 500° C. and lower than or equal to 950° C. inthe heating furnace and the soaking furnace; and

subjecting the base steel material to an alloying treatment at higherthan or equal to 440° C. and lower than or equal to 600° C.,

wherein the annealing treatment is performed under the followingconditions:

-   -   conditions of the heating furnace: an all radiant tube type        heating furnace is used, a time period that the temperature of        the base steel material is in the range of higher than or equal        to 500° C. and lower than or equal to 950° C. is 100 seconds to        1000 seconds, an atmosphere of the heating furnace contains        hydrogen, water vapor, and nitrogen, a logarithm        log(P_(H2O)/P_(H2)) of a value obtained by dividing a partial        water vapor pressure (P_(H2O)) by a partial hydrogen pressure        (P_(H2)) is more than or equal to −4.0 and less than −2.0, and a        hydrogen concentration is more than or equal to 3 vol % and less        than or equal to 30 vol %; and    -   conditions of the soaking furnace: a time period that the        temperature of the base steel material is in the range of higher        than or equal to 500° C. and lower than or equal to 950° C. is        100 seconds to 1000 seconds, an atmosphere of the soaking        furnace contains hydrogen, water vapor, and nitrogen, a        logarithm log(P_(H2O)/P_(H2)) of a value obtained by dividing a        partial water vapor pressure (P_(H2O)) by a partial hydrogen        pressure (P_(H2)) is more than or equal to −8.0 and less than        −4.0, and a hydrogen concentration is more than or equal to 3        vol % and less than or equal to 30 vol %.

-   [5] A method of manufacturing the alloyed hot-dip galvanized steel    sheet according to [4],

wherein the base steel material further contains one or more of, in mass%,

Cr: more than or equal to 0.05% and less than or equal to 1.0%,

Ni: more than or equal to 0.05% and less than or equal to 1.0%,

Cu: more than or equal to 0.05% and less than or equal to 1.0%,

Nb: more than or equal to 0.005% and less than or equal to 0.3%,

Ti: more than or equal to 0.005% and less than or equal to 0.3%,

V: more than or equal to 0.005% and less than or equal to 0.5%,

B: more than or equal to 0.0001% and less than or equal to 0.01%,

Ca: more than or equal to 0.0005% and less than or equal to 0.04%,

Mg: more than or equal to 0.0005% and less than or equal to 0.04%,

La: more than or equal to 0.0005% and less than or equal to 0.04%,

Ce: more than or equal to 0.0005% and less than or equal to 0.04%, and

Y: more than or equal to 0.0005% and less than or equal to 0.04%.

Effect(s) of the Invention

According to the present invention, there is provided the high-strengthalloyed hot-dip galvanized steel sheet including the alloyed hot-dipgalvanized layer having excellent wettability of plating and adhesion ofplated layer on the base steel sheet containing C, Si, and Mn and havinga tensile strength of 590 MPa or more.

BRIEF DESCRIPTION OF THE DRAWING(S)

FIG. 1 is a graph showing a relationship of an Fe content in an alloyedhot-dip galvanized layer and a thickness of the alloyed hot-dipgalvanized layer to wettability of plating and adhesion of plated layer,which is obtained from results of Examples and Comparative Examples ofthe present invention to be described later.

FIG. 2 is a graph showing a relationship of a log(P_(H2O)/P_(H2)) of aheating furnace and a ferrite structure content in an A layer towettability of plating and adhesion of plated layer, which is obtainedfrom results of Examples and Comparative Examples of the presentinvention to be described later.

FIG. 3 is a graph showing a relationship of a log(P_(H2O)/P_(H2)) of asoaking furnace and a content of unoxidized Fe in an A layer towettability of plating and adhesion of plated layer, which is obtainedfrom results of Examples and Comparative Examples of the presentinvention to be described later.

FIG. 4 is a graph showing a relationship of a log(P_(H2O)/P_(H2)) of asoaking furnace and a total of contents of oxides of Fe, Si, Mn, P, S,and Al in an A layer to wettability of plating and adhesion of platedlayer, which is obtained from results of Examples and ComparativeExamples of the present invention to be described later.

FIG. 5 is a graph showing a relationship of a log(P_(H2O)/P_(H2)) of aheating furnace and a C content in an A layer to wettability of platingand adhesion of plated layer, which is obtained from results of Examplesand Comparative Examples of the present invention to be described later.

FIG. 6 is a graph showing a relationship of a log(P_(H2O)/P_(H2)) of aheating furnace and a thickness of an A layer to wettability of platingand adhesion of plated layer, which is obtained from results of Examplesand Comparative Examples of the present invention to be described later.

FIG. 7 is a graph showing a relationship of maximum sheet temperature ofa heating furnace and a time period that temperature of a cold-rolledsteel sheet is in a range of higher than or equal to 500° C. and lowerthan or equal to 950° C. in the heating furnace to wettability ofplating and adhesion of plated layer, which is obtained from results ofExamples and Comparative Examples of the present invention to bedescribed later.

FIG. 8 is a graph showing a relationship of maximum sheet temperature ofa soaking furnace and a time period that temperature of a cold-rolledsteel sheet is in a range of higher than or equal to 500° C. and lowerthan or equal to 950° C. in the soaking furnace to wettability ofplating and adhesion of plated layer, which is obtained from results ofExamples and Comparative Examples of the present invention to bedescribed later.

FIG. 9 is a graph showing a relationship of a log(P_(H2O)/P_(H2)) of aheating furnace and a log(P_(H2O)/P_(H2)) of a soaking furnace towettability of plating and adhesion of plated layer, which is obtainedfrom results of Examples and Comparative Examples of the presentinvention to be described later.

FIG. 10 is a graph showing a relationship of a hydrogen concentration ina heating furnace and a hydrogen concentration in a soaking furnace towettability of plating and adhesion of plated layer, which is obtainedfrom results of Examples and Comparative Examples of the presentinvention to be described later.

FIG. 11 is a graph showing a relationship of alloying temperature in analloying treatment and an Fe content in an alloyed hot-dip galvanizedlayer to wettability of plating and adhesion of plated layer, which isobtained from results of Examples and Comparative Examples of thepresent invention to be described later.

MODE(S) FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be described in detail.

First, let us assume that steel components of the base steel sheetincluding the alloyed hot-dip galvanized layer according to the presentinvention are as follows, and in addition, the base steel sheet has atensile strength of 590 MPa or more. Note that “%” used for the steelcomponents described in the following description represents “mass %”unless otherwise particularly explained.

C: C is an element which can increase the strength of the base steelsheet. However, when the content is less than 0.05%, it is difficult toachieve both of the tensile strength of 590 MPa or more and theworkability. On the other hand, when the content exceeds 0.50%, it isdifficult to ensure the spot weldability. For this reason, the range isset to more than or equal to 0.05% and less than or equal to 0.50%.

Si: Si is a strengthening element and is effective for increasing thestrength of the base steel sheet. Si can suppress precipitation ofcementite. When the content is less than 0.2%, the effect of highstrengthening is small. On the other hand, when the content exceeds3.0%, the workability is decreased. Accordingly, the content of Si isset to the range of more than or equal to 0.2% and less than or equal to3.0%.

Mn: Mn is a strengthening element and is effective for increasing thestrength of the base steel sheet. However, when the content is less than0.5%, it is difficult to obtain the tensile strength of 590 MPa or more.Conversely, when the content is a large quantity, it facilitatesco-segregation with P and S and leads to a remarkable deterioration inthe workability, and thus the upper limit is 5.0%. Accordingly, thecontent of Mn is set to the range of more than or equal to 0.5% and lessthan or equal to 5.0%.

Al: Al promotes the formation of ferrite, and improves the ductility. Alcan also act as a deoxidizing material. The effects thereof areinsufficient when the content is less than 0.001%. On the other hand,excessive addition increases the number of Al-based coarse inclusions,which can cause the deterioration in hole expandability as well assurface defects. Accordingly, the content of Al is set to more than orequal to 0.001% and less than or equal to 1.0%.

P: P tends to segregate at the center part of thickness of the steelsheet and causes the weld zone to become brittle. When the contentexceeds 0.1%, the embrittlement of the weld zone becomes remarkable, sothe suitable range is set to less than or equal to 0.1%. That is, P isregarded as an impurity and is limited to less than or equal to 0.1%.The lower limit value of P is not particularly determined, but when thelower limit is less than 0.0001%, it is disadvantageous economically, sothis value is preferably set to the lower limit value.

S: S has an adverse effect on the weldability and on themanufacturability at the time of casting and hot rolling. For thisreason, the upper limit value is less than or equal to 0.01%. That is, Sis regarded as an impurity and is limited to less than or equal to0.01%. The lower limit value of S is not particularly determined, butwhen the lower limit is less than 0.0001%, it is disadvantageouseconomically, so this value is preferably set to the lower limit value.Since S combines with Mn to form coarse MnS, which deteriorates thebendability and the hole expandability, it is preferred that the contentof S be reduced as much as possible.

N: N forms coarse nitrides and causes the deterioration of thebendability and hole expandability, so it is necessary to restrict theadditive amount. This is because when the content of N exceeds 0.01%,the above tendency becomes remarkable, so N is regarded as an impurityand the content of N is in a range of less than or equal to 0.01%. Theeffect of the present invention is exhibited without particularlylimiting the lower limit, but when the content of N is less than0.0005%, the manufacturing cost dramatically increases, so this value isa substantial lower limit.

The base steel sheet according to the present invention may furtherinclude, as necessary, one or more selected from the group consisting ofCr, Ni, Cu, Nb, Ti, V, B, Ca, Mg, La, Ce, and Y.

Cr: Cr is a strengthening element and is important for improvement ofhardenability. However, when the content is less than 0.05%, theseeffects cannot be obtained, so, in the case of including Cr, the lowerlimit value is set to 0.05%. Conversely, when the content exceeds 1.0%,it has an adverse effect on the manufacturability at the time ofmanufacturing and hot rolling, so the upper limit value is set to 1.0%.

Ni: Ni is a strengthening element and is important for improvement ofhardenability. However, when the content is less than 0.05%, theseeffects cannot be obtained, so, in the case of including Ni, the lowerlimit value is set to 0.05%. Conversely, when the content exceeds 1.0%,it has an adverse effect on the manufacturability at the time ofmanufacturing and hot rolling, so the upper limit value is set to 1.0%.

Cu: Cu is a strengthening element and is important for improvement ofhardenability. However, when the content is less than 0.05%, theseeffects cannot be obtained, so, in the case of including Cu, the lowerlimit value is set to 0.05%. Conversely, when the content exceeds 1.0%,it has an adverse effect on the manufacturability at the time ofmanufacturing and hot rolling, so the upper limit value is set to 1.0%.

Nb: Nb is a strengthening element. It helps to increase the strength ofthe base steel sheet through the precipitate strengthening, thegrain-refining strengthening due to the growth inhibition of ferritecrystal grains, and the dislocation strengthening due to the inhibitionof recrystallization. When the additive amount is less than 0.005%,these effects cannot be obtained, so, in the case of including Nb, thelower limit value is set to 0.005%. When the content exceeds 0.3%, thecarbonitride precipitation increases and the formability tends todeteriorate, so the upper limit is set to 0.3%.

Ti: Ti is a strengthening element. It helps to increase the strength ofthe base steel sheet through precipitate strengthening, grain-refiningstrengthening due to the growth inhibition of ferrite crystal grains,and dislocation strengthening due to the inhibition ofrecrystallization. When the additive amount is less than 0.005%, theseeffects cannot be obtained, so, in the case of including Ti, the lowerlimit value is set to 0.005%. When the content exceeds 0.3%,carbonitride precipitation increases and the formability tends todeteriorate, so the upper limit is set to 0.3%.

V: V is a strengthening element. It helps to increase the strength ofthe steel sheet through the precipitate strengthening, thegrain-refining strengthening due to the growth inhibition of ferritecrystal grains, and the dislocation strengthening due to the inhibitionof recrystallization. When the additive amount is less than 0.005%,these effects cannot be obtained, so, in the case of including V, thelower limit value is set to 0.005%. When the content exceeds 0.5%, thecarbonitride precipitation increases and the formability tends todeteriorate, so the upper limit is set to 0.5%.

B: B is effective for grain boundary strengthening and steelstrengthening by addition of more than or equal to 0.0001%, but when theadditive amount thereof exceeds 0.01%, not only the effect of additionbecomes saturated, but the manufacturability at the time of hot rollingis decreased, so the upper limit thereof is set to 0.01%.

Ca, Mg, La, Ce, and Y may each be included more than or equal to 0.0005%and less than or equal to 0.04%. Ca, Mg, La, Ce, and Y are elements usedfor deoxidation, and it is preferred that the content of each of theelements be more than or equal to 0.0005%. However, when the contentexceeds 0.04%, this may cause deterioration of the formability.Accordingly, the content of each of the elements is set to more than orequal to 0.0005% and less than or equal to 0.04%.

Note that, in the present invention, La, Ce, and Y are generally addedin a mischmetal, which in addition to La and Ce may also contain otherlanthanoid series elements in combination. The effects of the presentinvention are exhibited even when the lanthanoid series elements otherthan La and Ce are contained as inevitable impurities. However, theeffects of the present invention are exhibited even when metals such asLa and Ce are added.

Next, the alloyed hot-dip galvanized layer according to the presentinvention will be described.

The alloyed hot-dip galvanized layer according to the present inventionis formed on a surface of the base steel sheet, which is a substrate,for ensuring corrosion prevention. Accordingly, in the presentinvention, the lowering of the adhesion of plated layer or thewettability of plating is a disadvantageous problem from the viewpointof ensuring the corrosion prevention.

As shown in FIG. 1, the alloyed hot-dip galvanized layer includes, inmass %, more than or equal to 5% and less than or equal to 15% of Fe,the balance including Zn and inevitable impurities.

When the Fe content is less than 5%, the amount of an Fe—Zn alloy phaseformed in the plated layer is small and the corrosion prevention isinsufficient. In addition, since slidability of the surface of theplated layer decreases, base steel sheet fracture or plated layerpeeling occurs at the time of performing press forming, and hence, theadhesion of plated layer deteriorates. When the Fe content exceeds 15%,in the Fe—Zn alloy phase formed in the plated layer, a Γ phase or a Γ1phase which is poor in ductility is formed with a large thickness. As aresult thereof, at the interface between the plated layer and thesubstrate steel sheet, the plated layer peels at the time of performingpress forming, and the corrosion prevention deteriorates. Note that theFe—Zn alloy phase used here represents all of the following: a ζ phase(FeZn₁₃), a δ₁ phase (FeZn₇), a Γ₁ phase (Fe₅Zn₂₁), and a Γ phase(Fe₃Zn₁₀).

Further, in the present invention, Al may further be included in theplated layer as necessary. With inclusion of more than or equal to 0.02%and less than or equal to 1.0% of Al in the plated layer, thewettability of plating and the adhesion of plated layer can be furtherenhanced.

A method of analyzing the Fe content per plated layer involves forexample: cutting an area of 30 mm×30 mm from the alloyed hot-dipgalvanized steel sheet; immersing the cut sample in 5% aqueous solutionof hydrochloric acid containing 0.02 vol % of inhibitor (IBIT 700A,manufactured by Asahi Chemical Co., Ltd); dissolving only the alloyedhot-dip galvanized layer; measuring the amount of Fe, the amount of Zn,and the amount of Al of the solution with ICP (ion plasma emissionanalyzer); and dividing the amount of Fe by the amount of Fe + theamount of Zn + the amount of Al and multiplying the result by 100. Inthe present invention, the Fe content represents an average of thevalues determined from five samples which are cut from locations thatare spaced apart from each other by 100 mm or more.

As shown in FIG. 1, the alloyed hot-dip galvanized layer has a thicknessof more than or equal to 3 μm and less than or equal to 30 μm.

The alloyed hot-dip galvanized layer having the thickness of less than 3μm is insufficient in the corrosion prevention. In addition, it becomesdifficult to uniformly form the plated layer on the base steel sheet,which may cause unplating, for example, and thus, the wettability ofplating deteriorates. The alloyed hot-dip galvanized layer having thethickness exceeding 30 μm is not economical, because the effect ofenhancing the corrosion prevention by the plated layer saturates. Inaddition, residual stress inside the plated layer increases, and theadhesion of plated layer deteriorates, for example, the plated layer maybe peeled at the time of performing press forming.

Regarding a method of measuring the thickness of the alloyed hot-dipgalvanized layer, there are various methods including the microscopiccross-section test method (JIS H 8501). This is a method of burying across-section of a sample in a resin, polishing it, then performingetching by a corrosive solution as necessary, and analyzing the polishedsurface by an optical microscope, a scan type electron microscope (SEM),an electron probe microanalyzer (EPMA), and the like, and finding thethickness. In the present invention, the sample was buried in Technovit4002 (manufactured by Maruto Instrument Co., Ltd.) and polished in orderby #240, #320, #400, #600, #800, and #1000 polishing paper (JIS R 6001),then the polished surface was analyzed by EPMA from the surface of theplated layer to the substrate steel sheet by line analysis. Then, thethickness at which Zn is no longer detected was found at positions ofany 10 locations that are spaced apart from each other by 1 mm or more,the found values are averaged, and the obtained value was determined tobe the thickness of the alloyed hot-dip galvanized layer.

Subsequently, an A layer, which is important in the present invention,will be described.

The alloyed hot-dip galvanized steel sheet according to the presentinvention includes the following A layer immediately under the surfaceof the base steel sheet, the A layer being formed in the base steelsheet and having a thickness of more than or equal to 2 μm and less thanor equal to 20 μm from the surface of the base steel sheet.

A layer: including more than or equal to 50 vol % of a ferrite structurebased on a volume of the A layer and the balance including inevitablestructures, and containing, based on a mass of the A layer, more than orequal to 90 mass % of unoxidized Fe, less than or equal to 10 mass % ofa total of contents of oxides of Fe, Si, Mn, P, S, and Al, and less than0.05 mass % of C.

The A layer according to the present invention is defined by thefollowing measurement method. Since the oxides of Fe, Si, Mn, P, S, andAl are decreased, the A layer is mainly composed of a ferrite structuresuppressed in C and excellent in ductility, which is different from alayer including internal oxides of Si and Mn or externally oxidized Siand Mn written in Patent Literatures or the like. Further, the A layeris a layer mainly composed of unoxidized Fe having high reactivity withzinc, and accurately controlled for improving wettability of plating andthe adhesion of plated layer. The alloyed hot-dip galvanized steel sheetincluding the A layer according to the present invention containing C,Si, Mn, and the like has a high-strength of 590 MPa or more, and isexcellent in the wettability of plating and the adhesion of platedlayer.

As shown in FIG. 2, it is necessary to include more than or equal to 50vol % of the ferrite structure based on a volume of the A layer forobtaining excellent adhesion of plated layer. The ferrite is a structureexcellent in ductility.

As described above, in the alloyed hot-dip galvanized steel sheet, theductility deteriorates with the increase in strength, and alongtherewith, pressing load at the time of performing press forming islarge, so that the shear stress applied to the plated layer from a moldat the time of performing forming increases. Accordingly, the platedlayer is easily peeled from the interface with the base steel sheet, andresults in defects in corrosion prevention and defects in appearancewith press scratches and the like, which may become a problem related tothe deterioration in the adhesion of plated layer. However, in thepresent invention, since the A layer immediately under the plated layerincludes a ferrite structure and is excellent in ductility, the problemis solved. If less than 50 vol % of the ferrite structure is included inthe A layer, the improvement in the adhesion of plated layer isinsufficient. It is preferred that the A layer include more than orequal to 55 vol % of the ferrite structure. The ferrite phase mayinclude a form of an acicular ferrite in addition to a polygonalferrite.

The inevitable structures included in the balance represent bainite,martensite, residual austenite, and pearlite.

Note that each phase of the structures such as ferrite, martensite,bainite, austenite, pearlite, and residual structures can be identifiedand their locations and area fraction can be observed and quantitativelymeasured using an optical microscope having a magnification of 1000times and a scanning and transmission electron microscope having amagnification of 1000 times to 100000 times after a cross section of thesteel sheet in a rolling direction or a cross section in the right angledirection of the rolling direction is etched using a Nital reagent andthe reagent as disclosed in JP 59-219473A. In Examples, the areafraction of the ferrite structure can be obtained by observing 20 ormore fields and applying the point-count method or image analysis up tothe depth of 2 μm from immediately under the surface of the base steelsheet. Then, the average value is determined as the content based on thevolume.

Further, it is necessary that the A layer include, based on a mass ofthe A layer, more than or equal to 90 mass % of unoxidized Fe, less thanor equal to 10 mass % of a total of contents of oxides of Fe, Si, Mn, P,S, and Al, and less than 0.05 mass % of C, for obtaining excellentwettability of plating and adhesion of plated layer.

As described above, in the high-strength alloyed hot-dip galvanizedsteel sheet including Si and Mn, Si and Mn are elements which are moreeasily oxidizable compared with Fe, so at the time of heating inrecrystallization annealing in a conventional Fe-reducing atmosphere, Siand Mn on the surface of the base steel sheet oxidize. Further, Si andMn which thermally diffuse from the inside of the base steel sheetoxidize at the surface whereby gradually the Si and Mn oxides becomeconcentrated on the surface. If the Si and Mn oxides concentrate at thesurface, in the process of dipping the base steel sheet in the hot-dipgalvanizing bath, contact between the molten zinc and the base steelsheet would be prevented, which would cause a problem of a drop in thewettability of plating and the adhesion of plated layer of the alloyedhot-dip galvanized layer. In addition, as described above, the internaloxides of Si and Mn written in Patent Literatures are also present inthe vicinity of the surface of the inside of the base steel sheet.Accordingly, there is a problem in that the ductility and thebendability of the base steel sheet are deteriorated and the pressforming cannot be performed. Further, when the shear stress is appliedto the plated layer at the time of performing the press forming, thereis a problem related to the adhesion of plated layer that the platedlayer peels from the vicinity of the surface of the inside of the basesteel sheet in which the internal oxides are present. However, in thepresent invention, the A layer immediately under the plated layer ismainly composed of Fe, and the oxides of Fe, Si, Mn, P, S, and Al aredecreased, so that the problems are solved. The oxides used here may beany of the internal oxides, or external oxides which concentrate on thesurface of the base steel sheet. Examples of the oxides include FeO,Fe₂O₃, Fe₃O₄, MnO, MnO₂, Mn₂O₃, Mn₃O₄, SiO₂, P₂O₅, Al₂O₃, SO₂ as singleoxides and respective nonstoichiometric compositions of single oxides,or FeSiO₃, Fe₂SiO₄, MnSiO₃, Mn₂SiO₄, AlMnO₃, Fe₂PO₃, Mn₂PO₃ as compositeoxides and respective nonstoichiometric compositions of compositeoxides.

For the reasons described above, as shown in FIG. 3, the improvement inthe wettability of plating and the adhesion of plated layer isinsufficient when the content of unoxidized Fe in the A layer is lessthan 90%. The content of Fe is preferably more than or equal to 92%.Further, as shown in FIG. 4, when the total of the contents of theoxides of Fe, Si, Mn, P, S, and Al exceeds 10%, the improvement in thewettability of plating and the adhesion of plated layer areinsufficient. The total of the contents of the oxides of Fe, Si, Mn, P,S, and Al is preferably less than or equal to 8%.

The content of unoxidized Fe in the A layer is determined as follows,for example. The alloyed hot-dip galvanized steel sheet is analyzed inthe depth direction using an X-ray photoelectron spectroscope with anion gun (XPS, PHI5800, manufactured by Ulvac Phi, Inc.), and the contentfrom the depth at which Zn could no longer be detected to the depth of 2μm further down, which is worked out from a zero-valent Fe spectrum, isaveraged by the depth. In the same manner, the total of the contents ofthe oxides of Fe, Si, Mn, P, S, and Al is determined by finding out therespective contents of Fe, Si, Mn, P, S, and Al from the depths at whichZn could no longer be detected to the depth of 2 μm further down, whichare worked out from Fe, Si, Mn, P, S, and Al spectra whose valences arenot zero, adding the contents, and then averaging the content by thedepth. However, the measurement method is not particularly limited, andthe contents may be determined using analysis means as necessary, suchas depth direction analysis using glow discharge spectrometry (GDS),secondary ion mass spectrometry (SIMS), and time-of-flight typesecondary ion mass spectrometry (TOF-SIMS), and cross-sectional analysisusing a transmission electron microscope (TEM) and an electron probemicroanalyzer (EPMA).

Further, as described above, in the high-strength alloyed hot-dipgalvanized steel sheet containing C, when C is present in a grainboundary or a grain of the base steel sheet in the recrystallizationannealing, there is a problem in that the reaction between the moltenzinc and the base steel sheet in the process of Fe—Zn alloying reactionafter dipping the base steel sheet in the hot-dip galvanizing bath isinhibited, to thereby deteriorate the adhesion of plated layer. Inaddition, there is also a problem in that the inclusion of C in thealloyed hot-dip galvanized layer after the alloying reaction lowers theductility of the plating, so that peeling of the plating easily occurswhen press forming is performed. However, in the present invention, thecontent of C in the A layer immediately under the plated layer isextremely reduced, and the problems are solved. For the reasonsdescribed above, as shown in FIG. 5, the improvement in the adhesion ofplated layer is insufficient when the content of C in the A layer ismore than or equal to 0.05%. The content of C in the A layer is lessthan 0.05%, and is preferably less than or equal to 0.03%.

The content of C in the A layer is determined as follows, for example.The alloyed hot-dip galvanized steel sheet is analyzed in the depthdirection using a GDS (GDA750, manufactured by Rigaku Corporation), andthe content from the depth at which Zn could no longer be detected tothe depth of 2 μm further down is averaged by the depth. However, themeasurement method is not particularly limited, and the contents may bedetermined using analysis means as necessary, such as depth directionanalysis using XPS, SIMS, and TOF-SIMS, and cross-sectional analysisusing TEM and EPMA.

As shown in FIG. 6, it is necessary that the A layer have a thickness ofmore than or equal to 2 μm and less than or equal to 20 μm for achievingexcellent wettability of plating and adhesion of plated layer. Theimprovement in the wettability of plating and the adhesion of platedlayer is insufficient when the thickness is less than 2 μm, and thestrength of the base steel sheet deteriorates when the thickness exceeds20 μm. The thickness of the A layer is preferably more than or equal to2 μm and less than or equal to 15 μm.

The thickness of the A layer is determined as follows. That is, vol % ofthe above-mentioned ferrite structure is measured from immediately underthe surface of the base steel sheet, and the depth at which the ferritestructure is less than 50 vol % (depth from immediately under thesurface of the base steel sheet) is represented by D1. D2 represents,when the steel sheet is analyzed in the depth direction using an XPS,the depth from the depth at which Zn could no longer be detected to thedepth at which the content of Fe is less than 90% determined by theabove-mentioned method. D3 represents the depth, which is determinedsimultaneously with D2 using the XPS, from the depth at which Zn couldno longer be detected to the depth at which the total of the contents ofFe, Si, Mn, P, S, and Al in the Fe, Si, Mn, P, S, and Al spectra whosevalences are not zero determined by the above-mentioned method exceeds10%. D4 represents, when the steel sheet is analyzed in the depthdirection using a GDS, the depth from the depth at which Zn could nolonger be detected to the depth at which the content of C determined bythe above-mentioned method is more than or equal to 0.05%. Then, amongaverage values D1(AVE) to D4(AVE) obtained by measuring five points ofeach of D1 to D4 at positions which are spaced apart from each other bymore than or equal to 20 mm and less than or equal to 50 mm, thesmallest value is employed as the thickness of the A layer. The thusdetermined A layer is a layer mainly composed of a ferrite structurecontaining Fe as a main component, which is decreased in the oxides ofFe, Si, Mn, P, S, and Al, which are external oxides or internal oxides,and is also decreased in C. As long as the A layer has a thicknesswithin the range of the present invention, the A layer is excellent inthe wettability of plating and the adhesion of plated layer.

Next, the method of manufacturing the alloyed hot-dip galvanized steelsheet according to the present invention will be described.

The manufacturing method includes subjecting a steel material containinggiven components to casting, hot-rolling, pickling, and cold rolling, tothereby produce a cold-rolled steel sheet (base steel sheet), subjectingthe cold-rolled steel sheet to an annealing treatment in a continuoushot-dip galvanizing plant equipped with a heating furnace and a soakingfurnace, and then performing a hot-dip galvanizing treatment and analloying treatment. In the heating furnace and the soaking furnace inwhich the annealing treatment is performed, the cold-rolled steel sheetwhose temperature is in the range of higher than or equal to 500° C. andlower than or equal to 950° C. while staying in the furnaces is passedunder the following conditions, and after that, the cold-rolled steelsheet is subjected to the hot-dip galvanizing treatment and subsequentlysubjected to the alloying treatment at an alloying heating temperatureof higher than or equal to 440° C. and lower than or equal to 600° C.Those conditions are important for manufacturing the alloyed hot-dipgalvanized steel sheet excellent in the wettability of plating and theadhesion of plated layer according to the present invention.

Conditions of the heating furnace: an all radiant tube type heatingfurnace is used, a time period that the temperature of the base steelmaterial is in the range of higher than or equal to 500° C. and lowerthan or equal to 950° C. is 100 seconds to 1000 seconds, an atmosphereof the heating furnace contains hydrogen, water vapor, and nitrogen, alogarithm log(P_(H2O)/P_(H2)) of a value obtained by dividing a partialwater vapor pressure (P_(H2O)) by a partial hydrogen pressure (P_(H2))is more than or equal to −4.0 and less than −2.0, and a hydrogenconcentration is more than or equal to 3 vol % and less than or equal to30 vol %.

Conditions of the soaking furnace: a time period that the temperature ofthe base steel material is in the range of higher than or equal to 500°C. and lower than or equal to 950° C. is 100 seconds to 1000 seconds, anatmosphere of the soaking furnace contains hydrogen, water vapor, andnitrogen, a logarithm log(P_(H2O)/P_(H2)) of a value obtained bydividing a partial water vapor pressure (P_(H2O)) by a partial hydrogenpressure (P_(H2)) is more than or equal to −8.0 and less than −4.0, anda hydrogen concentration is more than or equal to 3 vol % and less thanor equal to 30 vol %.

In the manufacturing method according to the present invention, theannealing treatment and the treatment of providing the plated layer isperformed in the continuous hot-dip galvanizing plant equipped with theall radiant tube type heating furnace. An all radiant tube type heatingfurnace is resistant to roll pickup and is good in productivity of theannealing treatment.

As shown in FIG. 7 and FIG. 8, regarding the conditions of the heatingfurnace and the conditions of the soaking furnace, it is necessary thatmaximum sheet temperature of the passing cold-rolled steel sheet behigher than or equal to 500° C. and lower than or equal to 950° C. formanufacturing the alloyed hot-dip galvanized steel sheet according tothe present invention. When the temperature is lower than 500° C., thetensile strength of the base steel sheet is lower than 590 MPa. Inaddition, naturally oxidized Fe on the surface of the base steel sheetremains after the annealing, to thereby deteriorate the wettability ofplating and the adhesion of plated layer. When the temperature exceeds950° C., excessive thermal energy is required, which is not economical.Further, since the volume fraction of ferrite decreases and the oxidesof Si and Mn are excessively formed, the wettability of plating and theadhesion of plated layer deteriorate. The temperature is preferablyhigher than or equal to 600° C. and lower than or equal to 850° C.

In the heating furnace, a log(P_(H2O)/P_(H2)) of the atmosphere in thefurnace is increased to oxidize C, Si, Mn, P, S, and Al on the surfaceof the base steel sheet. If C is oxidized, C detaches from the basesteel sheet as carbon monoxide or carbon dioxide, and hence, the Ccontent on the surface of the base steel sheet can be decreased.Further, Si, Mn, P, S, and Al are internally oxidized immediately underthe surface of the base steel sheet. At that time, by controlling thelevel of the log(P_(H2O)/P_(H2)) appropriately, the oxidation of Fe canbe suppressed. Accordingly, the excellent wettability of plating andadhesion of plated layer can be obtained.

As shown in FIG. 7, in the heating furnace, the time period that thetemperature of the base steel material is in the range of higher than orequal to 500° C. and lower than or equal to 950° C. is 100 seconds to1000 seconds. When the time period is less than 100 seconds, thedecreased amount of the C content and the amount of internally oxidizedSi, Mn, P, S, and Al are small, and hence, the wettability of platingand the adhesion of plated layer deteriorate. When the time periodexceeds 1000 seconds, the productivity deteriorates, and the C contentis excessively decreased to cause lowering in the tensile strength andto deteriorate the adhesion of plated layer due to excessive internaloxidization and generation of internal stress.

As shown in FIG. 9, in the heating furnace, the atmosphere in which thebase steel sheet is in the range of higher than or equal to 500° C. andlower than or equal to 950° C. contains hydrogen, water vapor, andnitrogen, and a logarithm log(P_(H2O)/P_(H2)) of a value obtained bydividing a partial water vapor pressure (P_(H2O)) by a partial hydrogenpressure (P_(H2)) is more than or equal to −4.0 and less than −2.0. Whenthe log(P_(H2O)/P_(H2)) is less than −4.0, the oxidation reaction of Cdoes not sufficiently proceed, and hence, the wettability of plating andthe adhesion of plated layer deteriorate. When the log(P_(H2O)/P_(H2))exceeds 0.0, since Fe oxides excessively form on the surface of thesteel sheet, the wettability of plating and the adhesion of plated layerdeteriorate. In addition, C in the base material is oxidized andexcessively detaches from the base material, which causes lowering inthe tensile strength of the base material, and internal stress of thesteel sheet increases due to excessive internal oxidization of Si, Mn,P, S, and Al, which causes deterioration in the adhesion of platedlayer. When the log(P_(H2O)/P_(H2)) is less than or equal to 0.0, thoseproblems can be avoided, but when the log(P_(H2O)/P_(H2)) is more thanor equal to −2.0, the deterioration of a lining of the heating furnace(normally manufactured by SUS Corporation) becomes noticeable, which isnot preferable in terms of industry. Accordingly, in the presentinvention, the log(P_(H2O)/P_(H2)) in the heating furnace is in therange of less than −2.0.

As shown in FIG. 10, the hydrogen concentration in the atmosphere of theheating furnace is more than or equal to 3 vol % and less than or equalto 30 vol %. When the hydrogen concentration is less than 3 vol %, it isdifficult to control the hydrogen concentration and thelog(P_(H2O)/P_(H2)) varies widely within the furnace. Therefore, thewettability of plating and the adhesion of plated layer deteriorate.When the hydrogen concentration exceeds 30 vol %, the amount of hydrogento be fed increases, which is not economical. In addition, hydrogenenters inside the steel sheet whereby hydrogen embrittlement occurs, andthe steel sheet strength and the adhesion of plated layer deteriorate.

Rate of temperature rise of the sheet in the heating furnace is notparticularly limited. However, if the rate is too slow, the productivitydeteriorates, and if the rate is too fast, the cost required for theheating plant becomes expensive. Accordingly, the rate is preferablymore than or equal to 0.5° C/s and less than or equal to 20° C/s.

Initial temperature of the sheet at the time of entering into theheating furnace is not particularly limited. However, if the temperatureis too high, Fe oxides are excessively formed on the base steel sheetand the wettability of plating and the adhesion of plated layerdeteriorate, and if the temperature is too low, cost required for thecooling becomes expensive. Accordingly, the temperature is preferablyhigher than or equal to 0° C. and lower than or equal to 200° C.

Subsequently, conditions of the soaking furnace continued from theheating furnace will be described.

In the soaking furnace, a log(P_(H2O)/P_(H2)) of the atmosphere in thefurnace is decreased to reduce the oxides that are formed by theinternal oxidization and external oxidization of Si, Mn, P, S, and Alimmediately under the surface of the base steel sheet formed in theheating furnace. With sufficient reduction, the excellent wettability ofplating and adhesion of plated layer can be obtained.

As shown in FIG. 8, in the soaking furnace, the time period that thetemperature of the steel sheet is in the range of higher than or equalto 500° C. and lower than or equal to 950° C. is 100 seconds to 1000seconds. When the time period is less than 100 seconds, the reduction ofthe oxides of Si, Mn, P, S, and Al is insufficient, and hence, thewettability of plating and the adhesion of plated layer deteriorate.When the time period exceeds 1000 seconds, the productivitydeteriorates, and the C content immediately under the surface of thebase steel sheet increases by thermal diffusion of C. Accordingly, thewettability of plating and the adhesion of plated layer deteriorate.

As shown in FIG. 9, in the soaking furnace, the atmosphere in which thesteel sheet is in the range of higher than or equal to 500° C. and lowerthan or equal to 950° C. contains hydrogen, water vapor, and nitrogen,and a logarithm log(P_(H2O)/P_(H2)) of a value obtained by dividing apartial water vapor pressure (P_(H2O)) by a partial hydrogen pressure(P_(H2)) is more than or equal to −8.0 and less than −4.0. When thelog(P_(H2O)/P_(H2)) is less than −8.0, in addition to that it is poor inindustrial practicality, in the case where ceramics are used for thefurnace body, the ceramics are reduced and lower the lifetime of thefurnace. When the log(P_(H2O)/P_(H2)) is more than or equal to −4.0, thereduction of Si, Mn, P, S, and Al is insufficient, and Si, Mn, and Alexternally oxidize, so that the wettability of plating and the adhesionof plated layer deteriorate. In addition, C in the base steel sheetdetaches from the base steel sheet by an oxidation reaction, whichcauses lowering in the tensile strength of the base steel sheet. Thelog(P_(H2O)/P_(H2)) of the atmosphere of the soaking furnace is morepreferably more than or equal to −7.0 and less than −4.0.

As shown in FIG. 10, the hydrogen concentration in the atmosphere of thesoaking furnace is more than or equal to 3 vol % and less than or equalto 30 vol %. When the hydrogen concentration is less than 3 vol %, it isdifficult to control the hydrogen concentration, and thelog(P_(H2O)/P_(H2)) varies widely within the furnace, so that thewettability of plating and the adhesion of plated layer deteriorate.When the hydrogen concentration exceeds 30 vol %, the amount of hydrogento be fed increases, which is not economical. In addition, hydrogenenters inside the steel sheet whereby hydrogen embrittlement occurs, andthe steel sheet strength and the adhesion of plated layer deteriorate.

Individual control of the atmospheric conditions in the heating furnaceand the soaking furnace of the continuous hot-dip galvanizing plant is acharacteristic feature of the method of manufacturing the hot-dipgalvanized steel sheet of the present invention. For individual control,it is necessary to charge the furnaces with nitrogen, water vapor, andhydrogen while controlling the concentrations thereof. Further, thelog(P_(H2O)/P_(H2)) of the oxygen potential in the heating furnace hasto be higher than the log(P_(H2O)/P_(H2)) of the oxygen potential in thesoaking furnace. For this reason, when gas flows from the heatingfurnace toward the soaking furnace, an additional atmosphere of a higherhydrogen concentration or lower water vapor concentration than theinside of the heating furnace may be introduced from between the heatingfurnace and the soaking furnace toward the soaking furnace. When gasflows from the soaking furnace toward the heating furnace, an additionalatmosphere of a lower hydrogen concentration or higher water vaporconcentration than the inside of the soaking furnace may be introducedfrom between the heating furnace and soaking furnace toward the heatingfurnace.

After the base steel sheet leaves the heating furnace and the soakingfurnace, the base steel sheet can be run through the general ordinarysteps until being dipped in the hot-dip galvanizing bath. For example,the base steel sheet can be run through a slow cooling step, a rapidcooling step, an overaging step, a second cooling step, a water quenchstep, a reheating step, and the like alone or in any combination. It isalso possible to similarly run the base steel sheet through generalordinary steps after dipping in a hot-dip galvanizing bath.

The base steel sheet is run through the heating furnace and the soakingfurnace, then is cooled and, in accordance with need, held intemperature, is dipped in a hot-dip galvanizing bath where it is hot-dipgalvanized, then is subjected to alloying treatment in accordance withneed.

With hot-dip galvanizing treatment, it is preferred to use a hot-dipgalvanizing bath which has a bath temperature of higher than or equal to440° C. and lower than 550° C., a concentration of Al in the bath ofmore than or equal to 0.08% and less than or equal to 0.24%, andinevitable impurities.

When the bath temperature is lower than 440° C., the molten zinc in thebath may solidify, so it becomes difficult to control the amount ofadhesion of the plating. When the bath temperature exceeds 550° C., theevaporation of the molten zinc at the bath surface becomes immense, theoperating cost rises, and vaporized zinc sticks to the inside of thefurnace, so there are problems in operation.

When the hot-dip galvanized steel sheet is subjected to the platingtreatment, if the concentration of Al in the bath becomes less than0.08%, a large amount of layer is formed and the adhesion of platedlayer deteriorates, while if the concentration of Al in the bath exceeds0.24%, the Al which oxidizes in the bath or on the bath increases andthe wettability of plating deteriorates.

As shown in FIG. 11, when performing hot-dip galvanizing treatment, thenalloying treatment, it is necessary that the alloying treatment beperformed at higher than or equal to 440° C. and lower than or equal to600° C. When the temperature is lower than 440° C., the alloyingproceeds slowly. When the temperature exceeds 600° C., due tooveralloying, a hard, brittle Zn—Fe alloy layer is overly formed at theinterface with the base steel sheet, and the adhesion of plated layerdeteriorates. Further, when the temperature exceeds 600° C., theresidual austenite phase of the base steel sheet breaks down, so thebalance of strength and ductility of the base steel sheet alsodeteriorates.

EXAMPLES

Hereinafter, the present invention will be specifically described by wayof Examples.

Test materials 1 to 94, which are shown in Tables 1 (Table 1-1, Table1-2), were prepared, the test materials 1 to 94 having been subjected tothe usual casting, hot-rolling, pickling, and cold rolling, and eachbeing a cold-rolled steel sheet (base steel sheet) having a thickness of1 mm. Some of the test materials 1 to 94 were appropriately selected andwere subjected to an annealing treatment, a hot-dip galvanizingtreatment, and an alloying treatment under the conditions of Tables 2and Tables 3, in a continuous hot-dip galvanizing plant equipped with anall radiant tube type heating furnace of a relatively high productivityheating method with little roll pickup as explained above. By using anall radiant tube type of furnace, as explained above, there is littleroll pickup and the productivity is also good.

After the soaking furnace, the base steel sheet was treated by generalslow cooling, rapid cooling, overaging, and second cooling steps andthen was dipped in a hot-dip galvanizing bath. The hot-dip galvanizingbath had a plating bath temperature of 460° C. and contained 0.13% of Aland 0.03% of Fe in addition to Zn. After the base steel sheet was dippedin the hot-dip galvanizing bath, the base steel sheet was wiped bynitrogen gas to adjust the plating thickness. After that, the base steelsheet was subjected to an alloying treatment by being heated in analloying furnace for 30 seconds. The obtained alloyed hot-dip galvanizedsteel sheet was evaluated for wettability of plating and adhesion ofplated layer. Tables 2 show the results of Examples, and Tables 3 showresults of Comparative Examples.

The wettability of plating was evaluated by mapping Zn and Fe on any 200μm×200 μm area of 10 locations that are spaced apart from each other by1 mm or more on the surface of the plated steel sheet of the alloyedhot-dip galvanized steel sheet by EPMA. The wettability of plating wasevaluated as follows. Regarding the case where there is no Zn and Fe isexposed, the case where there are four or more locations out of 10locations was evaluated as poor in the wettability of plating (Poor),the case where there are one to three locations out of 10 locations wasevaluated as good in the wettability of plating (Good), and the casewhere no such location was evaluated as excellent in the wettability ofplating (Excellent). “Good” and “Excellent” were each evaluated as passin the wettability of plating and “Poor” was evaluated as fail in thewettability of plating.

The adhesion of plated layer was measured by a powdering test. The casewhere a peeled length exceeds 2 mm was evaluated as poor in the adhesionof plated layer (Poor), the case where a peeled length was less than orequal to 2 mm and more than 1 mm was evaluated as good in the adhesionof plated layer (Good), and the case where a peeled length was less thanor equal to 1 mm was evaluated as excellent in the adhesion of platedlayer (Excellent). The powdering test is a method of examination ofadhesion involving sticking Cellotape (registered trademark) to thealloyed hot-dip galvanized steel sheet, bending the tape surface at R=1,90° C., unbending the tape, then peeling off the tape, and measuring thepeeled length of the alloyed hot-dip galvanized steel sheet.

A tensile test was performed by sampling a JIS No. 5 test piece from analloyed hot-dip galvanized steel sheet having a thickness of 1.0 mm indirections vertical to and parallel to the rolling direction to evaluatetensile properties. The tensile test was performed on each of five testpieces in the vertical direction and in the parallel direction, and anaverage value of the results was determined as a tensile strength (TS).Note that, as for a steel sheet having large material anisotropy, therewas a tendency that the elongation values varied.

As shown in Tables 2 (Table 2-1, Table 2-2, Table 2-3, and Table 2-4)and Tables 3 (Table 3-1 and Table 3-2), it was found out that thewettability of plating and the adhesion of plated layer of Examples(Tables 2) according to the present invention were excellent compared toComparative Examples (Tables 3). Note that, when the log(P_(H2O)/P_(H2))in the heating furnace is in the range of more than or equal to −4.0 andless than or equal to 0.0, the wettability of plating and the adhesionof plated layer were better compared to Comparative Example, but whenthe log(P_(H2O)/P_(H2)) is more than or equal to −2.0, the deteriorationof a lining of the heating furnace (normally manufactured by SUSCorporation) became noticeable.

TABLE 1-1 Composition [wt %] No. C Si Mn P S Al N Other selectedelement(s) Test material 1 0.06 0.5 2.5 0.050 0.004 0.20 0.002 Testmaterial 2 0.06 0.5 3.5 0.050 0.004 0.20 0.002 Test material 3 0.06 0.34.5 0.050 0.004 0.20 0.002 Test material 4 0.06 1.0 2.5 0.050 0.004 0.200.002 Test material 5 0.06 1.0 3.5 0.050 0.004 0.20 0.002 Test material6 0.06 1.0 4.5 0.050 0.004 0.20 0.002 Test material 7 0.06 1.5 0.5 0.0500.004 0.20 0.002 Test material 8 0.06 1.5 3.5 0.050 0.004 0.20 0.002Test material 9 0.06 1.5 4.5 0.050 0.004 0.20 0.002 Test material 100.06 2.5 0.5 0.050 0.004 0.20 0.002 Test material 11 0.06 2.5 1.5 0.0500.004 0.20 0.002 Test material 12 0.06 2.5 2.5 0.050 0.004 0.20 0.002Test material 13 0.06 2.5 3.5 0.050 0.004 0.20 0.002 Test material 140.06 2.5 4.5 0.050 0.004 0.20 0.002 Test material 15 0.1 0.5 0.5 0.0050.001 0.04 0.002 Test material 16 0.1 0.5 1.5 0.005 0.001 0.04 0.002Test material 17 0.1 0.5 2.5 0.005 0.001 0.04 0.002 Test material 18 0.10.5 3.5 0.005 0.001 0.04 0.002 Test material 19 0.1 0.5 4.5 0.005 0.0010.04 0.002 Test material 20 0.1 1.0 0.5 0.005 0.001 0.04 0.002 Testmaterial 21 0.1 1.0 1.5 0.005 0.001 0.04 0.002 Test material 22 0.1 1.02.5 0.005 0.001 0.04 0.002 Test material 23 0.1 1.0 3.5 0.005 0.001 0.040.002 Test material 24 0.1 1.0 4.5 0.005 0.001 0.04 0.004 Test material25 0.1 1.5 0.5 0.005 0.001 0.04 0.004 Test material 26 0.1 1.5 1.5 0.0050.001 0.04 0.004 Test material 27 0.1 1.5 2.5 0.005 0.001 0.04 0.004Test material 28 0.1 1.5 3.5 0.005 0.001 0.04 0.004 Test material 29 0.11.5 4.5 0.005 0.001 0.04 0.004 Test material 30 0.1 2.5 0.5 0.005 0.0010.04 0.004 Test material 31 0.1 2.5 1.5 0.005 0.001 0.04 0.004 Testmaterial 32 0.1 2.5 2.5 0.005 0.001 0.04 0.004 Test material 33 0.1 2.53.5 0.005 0.001 0.04 0.004 Test material 34 0.1 2.5 4.5 0.005 0.001 0.040.002 Test material 35 0.2 0.5 0.5 0.001 0.0005 0.01 0.002 Test material36 0.2 0.3 1.5 0.001 0.0005 0.01 0.002 Test material 37 0.2 0.3 2.50.001 0.0005 0.01 0.002 Test material 38 0.2 0.5 3.5 0.001 0.0005 0.010.002 Test material 39 0.2 0.5 4.5 0.001 0.0005 0.01 0.002 Test material40 0.2 1.0 0.5 0.001 0.0005 0.01 0.002 Test material 41 0.2 1.0 1.50.001 0.0005 0.01 0.002 Test material 42 0.2 1.0 2.5 0.001 0.0005 0.010.002 Test material 43 0.2 1.0 3.5 0.001 0.0005 0.01 0.002 Test material44 0.2 1.0 4.5 0.001 0.0005 0.01 0.002 Test material 45 0.2 1.5 0.50.001 0.0005 0.01 0.002 Test material 46 0.2 1.5 1.5 0.001 0.0005 0.010.002 Test material 47 0.2 1.5 2.5 0.001 0.0005 0.01 0.002

TABLE 1-2 Composition [wt %] No. C Si Mn P S Al N Other selectedelement(s) Test material 48 0.2 1.5 3.5 0.001 0.0005 0.01 0.002 Testmaterial 49 0.2 1.5 4.5 0.001 0.0005 0.01 0.002 Test material 50 0.2 2.50.5 0.001 0.0005 0.01 0.002 Test material 51 0.2 2.5 1.5 0.001 0.00050.01 0.002 Test material 52 0.2 2.5 2.5 0.001 0.0005 0.01 0.002 Testmaterial 53 0.2 2.5 3.5 0.001 0.0005 0.01 0.002 Test material 54 0.2 2.54.5 0.001 0.0005 0.01 0.002 Test material 55 0.4 0.5 0.5 0.001 0.0010.005 0.002 Test material 56 0.4 0.5 1.5 0.001 0.001 0.005 0.002 Testmaterial 57 0.4 0.5 2.5 0.001 0.001 0.005 0.002 Test material 58 0.4 0.53.5 0.001 0.001 0.005 0.002 Test material 59 0.4 0.5 4.5 0.001 0.0010.005 0.002 Test material 60 0.4 1.0 0.5 0.001 0.001 0.005 0.002 Testmaterial 61 0.4 1.0 1.5 0.001 0.001 0.005 0.002 Test material 62 0.4 1.02.5 0.001 0.001 0.005 0.002 Test material 63 0.4 1.0 3.5 0.001 0.0010.005 0.002 Test material 64 0.4 1.0 4.5 0.001 0.001 0.005 0.002 Testmaterial 65 0.4 1.5 0.5 0.001 0.001 0.005 0.002 Test material 66 0.4 1.51.5 0.001 0.001 0.005 0.002 Test material 67 0.4 1.5 2.5 0.001 0.0010.005 0.002 Test material 68 0.4 1.5 3.5 0.001 0.001 0.005 0.002 Testmaterial 69 0.4 1.5 4.5 0.001 0.001 0.005 0.002 Test material 70 0.4 2.50.5 0.001 0.001 0.005 0.002 Test material 71 0.4 2.5 1.5 0.001 0.0010.005 0.002 Test material 72 0.4 2.5 2.5 0.001 0.001 0.005 0.002 Testmaterial 73 0.4 2.5 3.5 0.001 0.001 0.005 0.002 Test material 74 0.4 2.54.5 0.001 0.001 0.005 0.002 Test material 75 0.2 1.5 2.5 0.005 0.0010.04 0.002 Cr: 0.2 Test material 76 0.2 1.5 2.5 0.005 0.001 0.04 0.002Ni: 0.2 Test material 77 0.2 1.5 2.5 0.005 0.001 0.04 0.002 Cu: 0.2 Testmaterial 78 0.2 1.5 2.5 0.005 0.001 0.04 0.002 Nb: 0.02 Test material 790.2 1.5 2.5 0.005 0.001 0.04 0.002 Ti: 0.02 Test material 80 0.2 1.5 2.50.005 0.001 0.04 0.002 V: 0.02 Test material 81 0.2 1.5 2.5 0.005 0.0010.04 0.002 B: 0.002 Test material 82 0.2 1.5 2.5 0.005 0.001 0.04 0.002Ca: 0.002 Test material 83 0.2 1.5 2.5 0.005 0.001 0.04 0.002 Mg: 0.002Test material 84 0.2 1.5 2.5 0.005 0.001 0.04 0.002 La: 0.002 Testmaterial 85 0.2 1.5 2.5 0.005 0.001 0.04 0.002 Ce: 0.002 Test material86 0.2 1.5 2.5 0.005 0.001 0.04 0.002 Y: 0.002 Test material 87 0.2 1.52.5 0.005 0.001 0.04 0.002 Cr: 0.1, Ni: 0.1 Test material 88 0.2 1.5 2.50.005 0.001 0.04 0.002 Cr: 0.1, B: 0.005 Test material 89 0.2 1.5 2.50.005 0.001 0.04 0.002 Cu: 0.1, Mg: 0.001 Test material 90 0.2 1.5 2.50.005 0.001 0.04 0.002 Nb: 0.001, Ti: 0.001 Test material 91 0.2 1.5 2.50.005 0.001 0.04 0.002 V: 0.01, La: 0.001 Test material 92 0.2 1.5 2.50.005 0.001 0.04 0.002 Cr: 0.5, Ce: 0.001 Test material 93 0.2 1.5 2.50.005 0.001 0.04 0.002 Cr: 0.1, Ti: 0.001, B: 0.0005 Test material 940.2 1.5 2.5 0.005 0.001 0.04 0.002 Cr: 0.7, Nb: 0.004, Ti: 0.004

TABLE 2-1 Recrystallization annealing conditions Heating furnaceconditions Soaking pit conditions Time Time period that period thattemperature temperature of cold- of cold- rolled steel rolled steelsheet is in sheet is in temperature temperature range of range of 500°C. to 500° C. to Cold- Maximum 950° C. in Maximum 950° C. in rolledsheet heating Oxygen Hydrogen sheet soaking Oxygen Hydrogen steeltemperature furnace potential concentration temperature pit potentialconcentration No. sheet [° C.] [sec] logP_(H2O)/P_(H2) [vol %] [° C.][sec] logP_(H2O)/P_(H2) [vol %] A1 Test material 1 675 253 −1.4 19 675297 −6.1 3 A2 Test material 3 852 442 −2.4 26 854 510 −5.2 27 A3 Testmaterial 4 813 356 −2.2 9 814 707 −7.3 15 A4 Test material 6 720 421−2.3 4 723 199 −6.1 6 A5 Test material 7 778 967 −1.5 15 780 421 −7.2 15A6 Test material 9 738 279 −3.0 19 739 251 −6.0 18 A7 Test material 10725 698 −2.7 20 725 274 −7.5 23 A8 Test material 13 678 487 −0.7 22 681444 −4.7 27 A9 Test material 14 716 218 −3.4 16 719 403 −6.7 23 A10 Testmaterial 15 616 144 −3.2 16 618 110 −7.4 16 A11 Test material 16 669 210−3.5 21 670 753 −6.5 24 A12 Test material 18 726 756 −1.5 22 728 560−7.4 9 A13 Test material 20 815 438 −2.6 28 816 714 −5.6 3 A14 Testmaterial 21 612 291 −1.5 5 614 703 −5.3 8 A15 Test material 22 754 462−2.9 11 756 604 −5.6 13 A16 Test material 24 638 157 −2.4 16 641 610−5.3 9 A17 Test material 26 826 442 −3.3 4 828 573 −7.3 26 A18 Testmaterial 27 856 725 −1.4 11 857 600 −4.2 18 A19 Test material 29 793 336−1.6 15 795 765 −6.4 22 A20 Test material 30 775 302 −2.5 24 777 613−6.1 4 A21 Test material 31 766 285 −2.6 18 768 257 −6.7 16 A22 Testmaterial 32 800 329 −3.1 29 801 609 −5.9 22 A23 Test material 34 843 319−1.3 16 844 299 −5.9 3 A24 Test material 36 826 856 −2.6 14 829 187 −6.310 A25 Test material 37 647 506 −2.9 22 649 397 −4.4 13 A26 Testmaterial 38 670 328 −0.8 15 671 182 −6.4 27 A27 Test material 39 736 716−3.2 13 738 645 −6.9 7 A28 Test material 40 634 275 −2.0 19 635 196 −5.026 A29 Test material 41 856 398 −2.1 12 859 813 −6.6 17 A30 Testmaterial 42 696 240 −2.7 5 697 465 −6.8 5 A31 Test material 43 899 666−1.8 15 901 251 −6.9 5 A32 Test material 44 686 357 −2.0 4 687 622 −4.710 A33 Test material 45 712 277 −2.1 15 712 315 −6.9 26 A34 Testmaterial 47 854 425 −1.8 15 857 195 −6.2 13 A35 Test material 49 625 361−2.2 26 626 467 −6.7 12 A36 Test material 50 717 228 −2.6 21 718 537−7.0 25 A37 Test material 51 858 506 −2.4 25 861 418 −4.5 20 A38 Testmaterial 52 748 468 −3.0 13 749 187 −4.8 18

TABLE 2-2 Recrystallization annealing conditions Heating furnaceconditions Soaking pit conditions Time Time period that period thattemperature temperature of cold- of cold- rolled steel rolled steelsheet is in sheet is in temperature temperature range of range of 500°C. to 500° C. to Cold- Maximum 950° C. in Maximum 950° C. in rolledsheet heating Oxygen Hydrogen sheet soaking Oxygen Hydrogen steeltemperature furnace potential concentration temperature pit potentialconcentration No. sheet [° C.] [sec] logP_(H2O)/P_(H2) [vol %] [° C.][sec] logP_(H2O)/P_(H2) [vol %] A39 Test material 53 672 201 −3.1 12 675430 −6.7 9 A40 Test material 54 812 409 −3.5 3 813 385 −6.0 13 A41 Testmaterial 55 883 531 −1.6 21 883 801 −5.4 23 A42 Test material 56 869 420−2.8 22 871 836 −6.3 12 A43 Test material 57 825 372 −2.3 18 826 724−6.0 23 A44 Test material 58 628 292 −3.4 6 630 473 −5.1 9 A45 Testmaterial 59 899 657 −2.7 3 900 536 −4.8 12 A46 Test material 60 631 154−1.8 21 631 477 −6.9 14 A47 Test material 61 716 230 −2.3 11 716 213−5.3 18 A48 Test material 63 729 305 −3.1 28 730 496 −6.9 23 A49 Testmaterial 64 818 500 −3.2 7 821 523 −6.0 13 A50 Test material 65 843 410−3.2 12 843 389 −4.6 7 A51 Test material 66 834 378 −0.9 17 836 673 −5.313 A52 Test material 67 702 368 −3.1 3 703 560 −4.8 14 A53 Test material68 708 320 −3.1 11 710 191 −4.9 27 A54 Test material 69 611 198 −3.1 8611 733 −4.9 23 A55 Test material 71 824 667 −1.1 16 826 576 −6.7 16 A56Test material 72 648 207 −1.4 19 651 199 −5.7 4 A57 Test material 74 767519 −1.6 23 769 600 −5.5 9 A58 Test material 76 842 606 −0.9 7 844 687−5.0 17 A59 Test material 77 882 368 −2.3 9 883 357 −6.8 22 A60 Testmaterial 78 894 824 −2.7 4 894 581 −6.0 12 A61 Test material 79 656 552−2.4 12 657 370 −6.0 27 A62 Test material 80 726 753 −2.1 19 727 203−4.6 15 A63 Test material 81 755 664 −3.3 7 756 411 −4.7 10 A64 Testmaterial 82 820 514 −0.8 21 820 223 −4.7 16 A65 Test material 83 888 781−0.9 22 890 857 −4.9 25 A66 Test material 84 699 315 −2.7 14 699 459−4.8 29 A67 Test material 85 614 338 −1.5 25 617 755 −6.1 16 A68 Testmaterial 86 634 171 −1.7 8 637 554 −4.9 11 A69 Test material 87 821 386−1.5 7 823 783 −4.5 11 A70 Test material 88 773 323 −0.7 24 774 785 −5.77 A71 Test material 89 841 444 −1.8 19 843 343 −4.2 4 A72 Test material90 803 374 −3.2 22 804 275 −4.6 24 A73 Test material 91 664 240 −2.3 13665 239 −6.4 29 A74 Test material 92 825 519 −1.0 27 827 294 −6.8 5 A75Test material 93 798 327 −3.2 24 799 399 −5.6 20 A76 Test material 94632 158 −2.9 12 634 176 −6.5 9

TABLE 2-3 A layer immediately under surface of substrate steel sheetAlloyed hot-dip Total of Evaluation Alloying galvanized layer Unoxidizedcontents of adhesion treatment Tensile Fe Ferrite Fe oxides of Fe,wettability of temperature strength content Thickness Thickness contentcontent Si, Mn, P, S, C of plated No. [° C.] [MPa] [wt %] [μm] [μm] [vol%] [wt %] and Al [wt %] content plating layer Remark A1 480 860 11 5 1166 95 3.4 0.012 Excellent Excellent Example A2 513 738 10 5 10 76 95 3.90.016 Excellent Excellent Example A3 562 668 11 4 11 55 92 6.3 0.017Good Good Example A4 568 807 12 5 12 54 93 5.5 0.008 Excellent ExcellentExample A5 537 830 11 6 19 82 91 7.5 0.009 Good Good Example A6 454 7397 8 7 56 93 6.8 0.017 Excellent Excellent Example A7 554 785 13 6 18 5393 5.9 0.016 Good Good Example A8 500 639 6 4 6 54 94 4.5 0.014Excellent Good Example A9 574 1067 11 5 11 58 93 6.7 0.006 ExcellentExcellent Example A10 476 1000 15 4 17 56 95 4.2 0.010 Good Good ExampleA11 459 727 9 6 9 55 96 1.9 0.017 Excellent Excellent Example A12 569608 13 10 13 58 92 5.8 0.010 Good Good Example A13 488 865 13 10 13 6393 6.2 0.022 Excellent Excellent Example A14 524 673 14 6 11 73 95 4.50.025 Excellent Excellent Example A15 510 752 9 3 9 68 91 7.4 0.017Excellent Good Example A16 475 961 15 7 18 59 96 2.2 0.020 ExcellentGood Example A17 477 912 14 8 17 51 96 2.4 0.011 Good Good Example A18536 809 14 4 13 88 92 6.4 0.020 Excellent Excellent Example A19 502 99812 6 12 79 91 7.4 0.014 Excellent Good Example A20 509 903 8 6 6 61 953.2 0.008 Excellent Excellent Example A21 520 1047 11 10 11 57 95 4.30.025 Excellent Good Example A22 500 638 13 4 13 57 94 3.6 0.016Excellent Excellent Example A23 578 716 14 8 14 95 95 3.9 0.015Excellent Excellent Example A24 581 906 6 5 5 78 95 4.7 0.024 ExcellentGood Example A25 535 1001 14 9 14 64 95 3.1 0.016 Excellent ExcellentExample A26 543 882 11 5 11 93 97 1.3 0.011 Excellent Good Example A27550 727 8 7 5 60 95 4.8 0.026 Good Excellent Example A28 477 830 12 7 1265 96 2.4 0.017 Excellent Good Example A29 543 847 9 5 9 69 92 6.8 0.015Excellent Excellent Example A30 526 695 10 5 10 76 92 7.3 0.015Excellent Excellent Example A31 570 1089 7 4 4 56 93 5.1 0.019 ExcellentGood Example A32 454 913 9 7 9 81 93 5.7 0.016 Excellent ExcellentExample A33 544 909 14 9 14 78 94 4.9 0.025 Excellent Good Example A34523 603 13 7 13 56 95 4.0 0.028 Excellent Good Example A35 460 717 7 6 360 94 5.7 0.025 Excellent Excellent Example A36 500 1027 13 7 13 55 972.3 0.021 Good Good Example A37 456 642 10 10 10 69 93 6.9 0.015Excellent Excellent Example A38 460 978 13 6 3 82 95 4.5 0.021 ExcellentGood Example

TABLE 2-4 A layer immediately under surface of substrate steel sheetAlloyed hot-dip Total of Evaluation Alloying galvanized layer contentsof adhesion treatment Tensile Fe Ferrite Unoxidized oxides of Fe,wettability of temperature strength content Thickness Thickness contentFe content Si, Mn, P, S, C of plated No. [° C.] [MPa] [wt %] [μm] [μm][vol %] [wt %] and Al [wt %] content plating layer Remark A39 507 787 148 14 63 93 6.8 0.016 Excellent Excellent Example A40 566 657 6 8 4 54 971.4 0.019 Excellent Good Example A41 550 906 14 8 13 83 93 5.4 0.033Excellent Excellent Example A42 556 662 12 8 12 62 93 7.3 0.024Excellent Excellent Example A43 537 796 11 5 11 69 95 3.9 0.023Excellent Good Example A44 453 954 15 8 14 58 92 6.4 0.023 ExcellentExcellent Example A45 501 714 7 22 7 62 93 4.9 0.023 Excellent GoodExample A46 501 759 11 17 11 74 94 4.7 0.030 Excellent Excellent ExampleA47 480 689 14 24 17 78 96 4.2 0.035 Excellent Good Example A48 584 101010 28 10 65 97 1.9 0.033 Excellent Excellent Example A49 566 812 14 1118 65 96 4.2 0.039 Excellent Good Example A50 471 1077 10 18 10 61 945.3 0.030 Excellent Excellent Example A51 548 1026 13 17 13 99 91 7.40.033 Excellent Good Example A52 523 867 12 21 12 65 95 3.3 0.035 GoodExcellent Example A53 524 595 8 10 5 60 93 5.2 0.039 Excellent GoodExample A54 498 907 9 12 9 63 94 5.4 0.032 Excellent Excellent ExampleA55 489 749 7 27 7 56 93 5.9 0.024 Excellent Good Example A56 452 843 1115 11 90 94 3.9 0.022 Excellent Excellent Example A57 515 732 9 13 9 5593 5.7 0.027 Excellent Excellent Example A58 485 914 13 9 13 70 93 7.30.017 Excellent Good Example A59 526 625 13 15 13 72 94 4.5 0.013Excellent Excellent Example A60 590 1055 12 10 12 60 92 7.5 0.022Excellent Good Example A61 528 944 11 12 11 65 95 5.1 0.020 ExcellentExcellent Example A62 585 981 8 26 8 57 95 3.8 0.019 Good Good ExampleA63 571 988 10 7 10 56 94 5.1 0.020 Excellent Excellent Example A64 5031083 9 17 9 67 95 3.8 0.015 Excellent Good Example A65 457 990 10 11 1068 94 4.0 0.023 Excellent Excellent Example A66 555 810 14 11 14 74 964.2 0.023 Excellent Excellent Example A67 578 826 14 13 14 54 96 3.60.012 Excellent Good Example A68 471 849 13 14 13 78 95 3.9 0.010Excellent Excellent Example A69 565 948 12 14 12 69 93 5.6 0.010Excellent Good Example A70 526 598 13 7 13 95 96 2.7 0.016 ExcellentExcellent Example A71 561 1007 14 10 14 71 92 6.6 0.028 Excellent GoodExample A72 530 771 7 20 3 63 92 6.8 0.015 Good Excellent Example A73538 705 8 20 5 77 93 6.5 0.011 Excellent Excellent Example A74 569 978 826 8 88 95 4.1 0.025 Excellent Good Example A75 570 967 14 16 14 82 926.7 0.018 Excellent Excellent Example A76 473 827 14 9 15 57 95 3.00.027 Excellent Good Example

TABLE 3-1 Recrystallization annealing conditions Heating furnaceconditions Soaking pit conditions Time Time period that period thattemperature temperature of cold- of cold- rolled steel rolled steelsheet is in sheet is in temperature temperature range of range of 500°C. to 500° C. to Maximum 950° C. in Maximum 950° C. in sheet heatingOxygen Hydrogen sheet soaking Oxygen Hydrogen Cold-rolled temperaturefurnace potential concentration temperature pit potential concentrationNo. steel sheet [° C.] [sec] logP_(H2O)/P_(H2) [vol %] [° C.] [sec]logP_(H2O)/P_(H2) [vol %] B1 Test material 1 479  0 −1.5 19 621 115 −6.519 B2 Test material 3 621 115 −1.6 15 433  0 −7.2 15 B3 Test material 5483  0 −4.2 24 485  0 −3.2 24 B4 Test material 7 963 313 −2.2  9 878 251−7.4  9 B5 Test material 9 921 856 −1.1 12 978 351 −6.8 12 B6 Testmaterial 11 991 1050  −2.3  6 989 542 −3.8  6 B7 Test material 13 778278   0.3 15 780 421 −6.0 15 B8 Test material 17 738 238 −4.3 19 739 251−7.0 19 B9 Test material 19 725 225 −1.3 20 725 274 −8.3 20 B10 Testmaterial 21 658 158   0.2 23 659 708 −8.5 23 B11 Test material 27 716216 −4.7 16 719 403 −3.2 16 B12 Test material 29 616 116 −3.2  1 618  70−7.4 10 B13 Test material 31 669 169 −3.5 35 670 753 −6.5  1 B14 Testmaterial 33 612 112 −1.0 32 615 242 −6.0 35 B15 Test material 35 726 226−1.5 12 728 560 −7.4  1 B16 Test material 37 778 278 −1.7  7 780 835−4.9 38 B17 Test material 39 815 315 −2.6 24 816 714 −5.6 24 B18 Testmaterial 41 612 112 −1.5  7 614 703 −5.3  7 B19 Test material 43 754 254−2.9 11 756 604 −5.6 11 B20 Test material 45 879 379 −1.6 17 881 761−6.4 17 B21 Test material 47 638 138 −2.4 16 641 610 −5.3 16 B22 Testmaterial 49 855 355 −0.5 10 856 711 −4.2 10 B23 Test material 51 826 326−3.3  6 828 573 −7.3  6 B24 Test material 53 856 356 −1.4 11 857 600−4.2 11 B25 Test material 55 782 282 −0.9 24 783 314 −4.3 24 B26 Testmaterial 57 793 293 −1.6 15 795 765 −6.4 15 B27 Test material 59 775 275−2.5 24 777 613 −6.1 24 B28 Test material 61 766  35 −1.5 18 768 257−6.7 18 B29 Test material 63 800  92   0.2  8 795 195 −5.9 25 B30 Testmaterial 65 793 1061    0.5 16 844 299 −6.5  8 B31 Test material 67 8431030  −1.3 22 701 315 −5.9 16 B32 Test material 69 700 1120  −2.6 14 8291011  −4.6 22 B33 Test material 71 826  79 −3.1 25 801 1097  −6.3 14 B34Test material 73 647  35 −4.5 15 671  91 −4.4 22 B35 Test material 75670  82 −4.7 19 635 1013  −6.4 15 B36 Test material 77 736 1013  −2.1 12859 1058  −6.9 13 B37 Test material 79 634 196 −2.7  8 697 1101  −5.0 19B38 Test material 81 856 147 −2.9 22 649  62 −6.6 12 B39 Test material83 696 236 −4.8 13 738  35 −6.8  8 Note: Underlined value is out ofrange of the present invention.

TABLE 3-2 A layer immediately under surface of substrate steel sheetAlloyed hot-dip Total of Alloying galvanized layer contents ofEvaluation treatment Tensile Fe Thick- Thick- Ferrite Unoxidized oxidesof Fe, wettability adhesion temperature strength content ness nesscontent Fe content Si, Mn, P, S, of of plated No. [° C.] [MPa] [wt %][μm] [μm] [vol %] [wt %] and Al [wt %] C content plating layer Remark B1480 567 11 5 0 35 86 12.0  0.052 Poor Poor Comparative Example B2 582561  8 4 0 42 85 13.0  0.055 Poor Poor Comparative Example B3 513 523 105 0 35 82 15.0  0.057 Poor Poor Comparative Example B4 562 668 11 4 0 5573 25.0  0.017 Poor Poor Comparative Example B5 554 743 11 4 0 51 7721.0  0.013 Poor Poor Comparative Example B6 568 807 12 5 0 54 84 15.0 0.008 Poor Poor Comparative Example B7 537 533 11 6 0 89 86 12.0  0.009Poor Poor Comparative Example B8 454 739  7 8 0 37 75 23.0  0.075 PoorPoor Comparative Example B9 554 785 13 6 0 64 77 21.0  0.080 Poor PoorComparative Example B10 690 664 10 7 0 76 75 23.0  0.019 Poor PoorComparative Example B11 574 1067  11 5 0 25 78 21.0  0.065 Poor PoorComparative Example B12 476 1000  15 4 15  56 94 4.2 0.010 Good PoorComparative Example B13 459 727  9 6 9 55 97 1.9 0.017 Good PoorComparative Example B14 516 868 10 5 10  84 92 5.1 0.008 Good PoorComparative Example B15 569 608 13 10  13  58 93 5.8 0.010 Good PoorComparative Example B16 551 1076  11 5 11  64 94 3.5 0.020 Good PoorComparative Example B17 435 865  2 10  13  63 91 6.2 0.022 Good PoorComparative Example B18 430 673  3 6 15  73 93 4.5 0.025 Good PoorComparative Example B19 620 752 17 3 9 68 90 7.4 0.017 Good PoorComparative Example B20 630 708 19 3 12  82 96 2.6 0.009 Good PoorComparative Example B21 660 961 18 7 15  59 95 2.2 0.020 Good PoorComparative Example B22 518 1021  18 2 12  78 94 4.1 0.020 Poor PoorComparative Example B23 477 912 17   1.5 14  58 96 2.4 0.011 Poor PoorComparative Example B24 536 809 20 1 14  88 92 5.8 0.020 Poor PoorComparative Example B25 547 641 10 40  9 89 92 6.2 0.020 Good PoorComparative Example B26 502 998  7 50  12  79 90 7.4 0.014 Good PoorComparative Example B27 509 903  8 35  8 53 95 3.2 0.008 Good PoorComparative Example B28 520 1047  11 10    0.5 57 94 4.3 0.025 Poor PoorComparative Example B29 500 638 13 4   0.4 57 95 3.6 0.016 Poor PoorComparative Example B30 511 757 13 7 40  61 95 2.7 0.007 Poor PoorComparative Example B31 578 716 14 8 25  95 95 3.9 0.015 Poor PoorComparative Example B32 496 699 11 4 30  63 91 6.3 0.013 Poor PoorComparative Example B33 581 906  6 5 27  52 93 4.7 0.024 Poor PoorComparative Example B34 535 1001  14 9 1 64 96 3.1 0.016 Poor PoorComparative Example B35 543 882 11 5   0.4 93 97 1.3 0.011 Poor PoorComparative Example B36 550 727  8 7 33  60 93 4.8 0.026 Poor PoorComparative Example B37 477 830 12 7 1 65 96 2.4 0.017 Poor PoorComparative Example B38 543 847  9 5   0.5 69 90 6.8 0.015 Poor PoorComparative Example B39 526 695 10 5   0.5 76 90 7.3 0.015 Poor PoorComparative Example Note: Underlined value is out of range of thepresent invention.

INDUSTRIAL APPLICABILITY

The alloyed hot-dip galvanized steel sheet manufactured using the methodaccording to the present invention has a high strength with a tensilestrength of 590 MPa or more, and has excellent wettability of platingand adhesion of plated layer. Accordingly, it is expected that thealloyed hot-dip galvanized steel sheet is applied as a material used inan automotive field, a household appliance field, and a buildingmaterial field.

The invention claimed is:
 1. A method of manufacturing an alloyedhot-dip galvanized steel sheet using a base steel material, the basesteel material containing, in mass %, C: more than or equal to 0.05% andless than or equal to 0.50%, Si: more than or equal to 0.2% and lessthan or equal to 3.0%, Mn: more than or equal to 0.5% and less than orequal to 5.0%, Al: more than or equal to 0.001 and less than or equal to1.0%, P: less than or equal to 0.1%, S: less than or equal to 0.01%, N:less than or equal to 0.01%, and the balance including Fe and inevitableimpurities, the method comprising: performing casting, hot-rolling,pickling, and cold rolling to thereby produce the base steel material;subjecting the base steel material to a hot-dip galvanizing treatment byperforming, using a continuous hot-dip galvanizing plant equipped with aheating furnace and a soaking furnace, an annealing treatment in which atemperature of the base steel material is increased within a range ofhigher than or equal to 500° C. and lower than or equal to 950° C. inthe heating furnace and the soaking furnace; and subjecting the basesteel material to an alloying treatment at higher than or equal to 440°C. and lower than or equal to 600° C., wherein the annealing treatmentis performed under the following conditions: conditions of the heatingfurnace: an all radiant tube type heating furnace is used, a time periodthat the temperature of the base steel material is in the range ofhigher than or equal to 500° C. and lower than or equal to 950° C. is100 seconds to 1000 seconds, an atmosphere of the heating furnacecontains hydrogen, water vapor, and nitrogen, a logarithmlog(P_(H2O)/P_(H2)) of a value obtained by dividing a partial watervapor pressure (P_(H2O)) by a partial hydrogen pressure (P_(H2)) is morethan or equal to −4.0 and less than −2.0, and a hydrogen concentrationis more than or equal to 3 vol % and less than or equal to 30 vol %; andconditions of the soaking furnace: a time period that the temperature ofthe base steel material is in the range of higher than or equal to 500°C. and lower than or equal to 950° C. is 100 seconds to 1000 seconds, anatmosphere of the soaking furnace contains hydrogen, water vapor, andnitrogen, a logarithm log(P_(H2O)/P_(H2)) of a value obtained bydividing a partial water vapor pressure (P_(H2O)) by a partial hydrogenpressure (P_(H2)) is more than or equal to −8.0 and less than −4.0, anda hydrogen concentration is more than or equal to 3 vol % and less thanor equal to 30 vol %.
 2. A method of manufacturing the alloyed hot-dipgalvanized steel sheet according to claim 1, wherein the base steelmaterial further contains one or more of, in mass %, Cr: more than orequal to 0.05% and less than or equal to 1.0%, Ni: more than or equal to0.05% and less than or equal to 1.0%, Cu: more than or equal to 0.05%and less than or equal to 1.0%, Nb: more than or equal to 0.005% andless than or equal to 0.3%, Ti: more than or equal to 0.005% and lessthan or equal to 0.3%, V: more than or equal to 0.005% and less than orequal to 0.5%, B: more than or equal to 0.0001% and less than or equalto 0.01%, Ca: more than or equal to 0.0005% and less than or equal to0.04%, Mg: more than or equal to 0.0005% and less than or equal to0.04%, La: more than or equal to 0.0005% and less than or equal to0.04%, Ce: more than or equal to 0.0005% and less than or equal to0.04%, and Y: more than or equal to 0.0005% and less than or equal to0.04%.